Continuous pipe, generally known within the oil and gas industry as coiled tubing because it is stored on a large reel, has been used for many years. It is much faster to run into and out of a well bore than conventional jointed straight pipe. There is no need to connect and disconnect short segments of straight pipe.
The only method by which a continuous length of tubing can be either forced against pressure into the well, or supported while hanging in the well bore or being lowered or raised, is by continuously gripping a length of the tubing just before it enters the well bore. This method is typically practiced by arranging two continuous chain loops with extended parallel sections on opposite sides of the tubing, in an opposing relationship. Each continuous chain carries a series of gripping elements. As each chain turns, the gripping elements come together on opposite sides of the tubing. A pair of skates, which include a long, straight and rigid beam that forces the grippers against the opposite sides of the tubing. The skates are pulled toward each other by hydraulic pistons or a similar mechanism to force the gripper elements against the tubing. Examples of coiled tubing injectors include those shown and described in U.S. Pat. No. 5,309,990, and U.S. applications Ser. Nos. 091070,592 and 09/070,593, all of which are incorporated herein by reference.
Coiled tubing has traditionally been used primarily for circulating fluids into the well and other work over operations, rather than drilling, because of its relatively small diameter and because it was not strong enough, especially for deep drilling. In recent years, however coiled tubing has been increasingly used to drill well bores. For drilling, a turbine motor is suspended at the end of the tubing and is driven by mud or drilling fluid pumped down the tubing. Coiled tubing has also been used as permanent tubing in production wells. These new uses of coiled tubing have been made possible by larger, stronger coiled tubing.
In order to handle larger, longer, and heavier tubing, the gripping force must be increased. Increased gripping force can be achieved by increasing the force pressing the gripper shoes against the tubing, the number of gripper shoes contacting the pipe, increasing the length of the chains, or the contact area of the gripper shoe. Increased gripping force can also be achieved through improving the gripping surfaces.
One problem with applying greater forces to the tubing is that dimensional variations of the components of the injector that are within manufacturing tolerances may nevertheless result in uneven gripping force applied to the tubing. For example, the position of a gripper element relative to the tubing may vary as the gripper elements moves on the injector's skate due to dimensional variations in the skate and rollers on which the gripper elements roll on the skate. Similarly, the position of the gripper element relative to the other gripper elements may also vary due to dimensional variations between the gripper elements, the elements of the chain and the elements used to attach the gripper element to the chain. The result of these dimensional variations is an uneven application of gripping force along the length of the tubing that is in the injector, resulting in less than maximum potential gripping force and less than satisfactory performance. The uneven application of force also results in excess stress placed on the tubing because all of the gripping force is being applied by certain grippers rather than distributed among all of the grippers. This excess force may cause undesirable deformation of the tubing. Excess stress and the strain associated with the deformation weakens the tubing and hastens its failure.